Tertiary olefins are in general commercially produced by the sulfuric acid extraction of such olefins from mixtures containing them obtained e.g., by steam cracking of petroleum feeds. Since this method uses sulfuric acid of high concentrations, the use of expensive materials for the apparatus is essential. Also, dilution of the acid to promote olefin recovery and reconcentrating the acid prior to recycling are required and are expensive. In addition this method is not always advantageous industrially, because tertiary olefins cause side reactions such as polymerization, hydration and the like during extraction with concentrated sulfuric acid.
It is also known that tertiary olefins may be prepared by reacting them selectively from such feeds with a primary alcohol in the presence of an acid catalyst to produce the corresponding alkyl tert-alkyl ethers. Only the tert-alkyl ethers are formed since the secondary olefins react very slowly and the primary olefins are completely inert. Such alkyl tert-alkyl ethers may then be easily separated and subsequently decomposed back to the tertiary olefins and the primary alcohol.
For producing tertiary olefins from alkyl tert-alkyl ethers, there have been proposed methods using various catalysts: For example aluminum compounds supported on silica or other carriers (U.S. Pat. No. 4,398,051). phosphoric acid on various supports (U.S. Pat. No. 4,320,232), metal containing weakly acidic components on a carrier of &gt;20 M.sup.2 /gm surface area (British Pat. No. 1,173,128). In addition inferior results are disclosed as being obtained utilizing carriers alone in the decomposition of methyl tertiary butyl ether (U.S. Pat. No. 4,398,051) and utilizing H.sub.2 SO.sub.4 treated clay in the decomposition of t-alkyl ether-alkanols (U.S. Pat. No. 4,254,290).
All these processes suffer from disadvantages. The most important disadvantages arise from the fact that appreciable amounts of dialkyl ether by-product are produced from the primary alcohol. This by-product is wasteful of primary alcohol feed since the dialkyl ether cannot be recovered and decomposed back to primary alcohol for recycle. Also, it causes considerable losses of tertiary olefin product in the final distillation due to the difficulty of separating the dialkyl ether from the tertiary olefin. For example an increase from 100 ppm to 1000 ppm by-product dialkyl ether formation causes substantial increases in olefin losses or capital costs for additional distillation facilities.
Other disadvantages are that the known catalysts do not have good catalysts life in that higher and higher temperatures which eventually become limiting are required to maintain high conversion of the alkyl tert-alkyl ethers. Additionally, larger amounts of the dialkyl ether by-product are produced as the catalyst ages with the disadvantage indicated above. This lack of good catalyst life may be due to the instability of the catalyst, to high temperature being required for good conversion thus promoting fouling, to the catalyst itself promoting fouling or to any or all of these. Also, a number of the catalysts such as resins cannot be regenerated after use.